Ellis has an early termination clause for the 2013-14 season, meaning basically he is free and clear to go where he wants after this season, and by many accounts, he is set to do so.
Jennings on the other hand is a restricted free agent next year, meaning the Bucks can match any offer he receives and have the cap space to do so.
There have been some rumblings recently between Jennings and the Bucks organization over them failing to give Jennings an extension already. However, the bottom line is if the Bucks want to keep Jennings, they are in a much better position to do so than they are with Ellis.
Also, it seems that at this point in his career that Ellis is regressing